The present invention concerns magnetic field sensing devices, magnetometers or gradiometers.
Today, in magnetic field detection or magnetic mass detection, relatively precise and efficient high-sensitivity sensors exist, for example, nuclear magnetic resonance magnetometers or superconductor magnetometers operating in liquid helium.
One obvious disadvantage of this type of magnetometer is its very high cost. Furthermore, superconductor magnetometers have a limited lifespan (it is not possible to maintain the low temperatures required for superconductors for very long periods under normal operating conditions).
Thus, fluxgate magnetometers which have the advantage of a relatively low cost are frequently used. The structure and operation of fluxgate magnetometers are described in relation to FIG. 1. It must be remembered that directional or nondirectional magnetometers are frequently used to detect earth field variations linked to the presence or passage of metallic objects or magnetic field sources, which requires the use of many devices.
FIG. 1 is a pictorial representation of a fluxgate magnetometer to explain the operating principles. This type of magnetometer, which is designed to be put in a magnetic field B, includes a saturable magnetic core 1 of high magnetic permeability. A winding 2, fed by an a.c. voltage V.sub.e, makes it possible to periodically bring the magnetic core into saturation.
When the magnetic core is saturated, magnetic field B is not substantially disturbed by the core, and the flux lines are, for example, straight as shown by the solid lines in FIG. 1. However, when the magnetic core is not saturated, the flux lines come closer together and tend to enter the magnetic core because of its high permeability. These flux lines are represented by the broken lines in FIG. 1.
A sense coil 3 placed near the core 1 which creates the disturbance in the magnetic field is a source of voltage V whose fundamental frequency corresponds to the excitation frequency. The voltage is characteristic of flux variations across winding 3 and thus is indirectly characteristic of the value of the field B in which the device is placed. In FIG. 1, the sense winding is represented as surrounding the saturable core 1. This is possible since the effect of opposite turns of the excitation winding 2 is cancelled. It should be noted, however, that the sense winding could be placed elsewhere in the field.
One disadvantage of such prior art magnetometers which have a relatively simple structure is their low sensitivity, no more than 100 microvolts/nT. This low sensitivity makes it necessary to use a long (high) sense coil with many turns. Thus, measurements of non-homogeneous fields result in errors linked to the influence of the field gradient (nondiagonal terms of the gradient tensor).
It is therefore an object of the present invention to provide a simply structured, high-sensitivity magnetic field sensor.
Another object of the present invention is to provide a magnetometer whose accuracy will not be significantly disturbed by possible field gradients.
Still another object of the present invention is to provide a magnetic field sensor whose accuracy will not be significantly disturbed by temperature variations and aging.
A further object of the present invention is to provide a magnetic field sensor that provides information directly in a digital form.
A still further object of the present invention is to provide such a sensor, that can be used as a simply structured and highly sensitive gradiometer.